Mastering Perl is the next book in the Learning Perl and Intermediate Perl progession of books.
I wrote the book "transparently", meaning just about anyone was free to take part, review the intermediate drafts, or make comments. You got to see how a book goes from the idea to the final product, and I got a better book as a result. You can read the proposal, which started the project, join the Mastering Perl mailing list that has infrequent updates on Mastering Perl new, or peruse the mailing list archives.
O'Reilly doesn't select an animal until the book goes into production, and I might not even get anything to say about it, but I'd like another camelid. The vicuña looks interesting, but the guanaco looks good too. Don't get too excited because there is no animal yet. I don't have a tricky name for it either. I can't call it the "MP book" because that's either Stas's mod_perl book or my military police book.